When performing a positioning control at an arbitrary halfway stopping position using an air pressure cylinder, a slight vibration of a piston tends to occur in the vicinity of a target position of the positioning or in the middle of movement for the positioning. This is an impediment of a high capability positioning control.
The cause of this slight vibration of the piston is considered as follows. In comparison with a control system of an electric positioning control system or an oil pressure positioning control system, the positioning control system using the air pressure cylinder (for example, the control system that is not provided with a slight-vibration-suppressing mechanism 20, in FIG. 1) has a characteristics of low rigidity and insufficient attenuation, and in addition, in a piston packing (Refer to numeral 5a in FIG. 1), turn and/or twist tend to occur in the vicinity of a neutral point. Accordingly, a large fluctuation of frictional resistance tends to occur, and the larger the coulomb friction and viscosity that occur at the packing portion are, the larger the effect of suppressing the slight vibration of the piston is, by nature. However, friction force of the piston packing of a large diameter is far larger than that of a rod packing of a small diameter in a usual cylinder, and this dominantly decides the friction characteristic of an entire drive system. Consequently, when the turn and the twist occur in the piston packing, the frictional resistance against the rod notably lowers, the effect of preventing the slight vibration lowers, and depending on a case, this serves as an encouragement for the vibration.
In relation to those problems, an air pressure cylinder whose piston rod has a variable damper using electric viscosity fluid attached thereto (refer to patent document 1), or a device that performs precise positioning by means of employing a static pressure bearing in a sliding portion (refer to patent document 2) are hitherto known. The former, i.e., the air pressure cylinder is provided with a damper piston having the electric viscosity fluid in the reciprocating piston rod and varies fluidity of the electric viscosity fluid by means of control of an electric field at the damper piston, and thereby the air pressure cylinder controls moving resistance of the rod. On the other hand, the latter, i.e., the positioning device uses a non-contact type piston, which is operated by means of a static pressure bearing and thereby improves positioning control accuracy and load control accuracy of a precise control device in a cylinder that feeds fluid being controlled via a servo valve into a pressure chamber.
However, in the conventional device that performs the positioning control by means of those systems, an equipment for controlling the frictional characteristics of the piston or the piston rod has to be large sized and complicated, and as a result, any one of those devices has to be manufactured at high costs. In addition, when the static pressure bearing is employed, there is little sliding friction and the damping effect due to the friction cannot be expected. Accordingly, the aforementioned slight vibration tends to easily occur. (Refer to FIG. 12 in the patent document 2)
Japanese Unexamined Patent Application Publication No. 11-287212
Japanese Unexamined Patent Application Publication No. 2004-144196